A solid-film-type film-coated sensor has long been desired in the fields of medicine and foods. Film-coated sensors of the coated wire electrode-type, in which a platinum electrode is directly coated with a polymeric film (e.g., a cellulose film), have recently appeared on the market. However, these sensors exhibit problems in terms of durability.
For example, in a solid-film-type oxygen sensor, the rate at which oxygen in a specimen solution is determined by an internal response electrode is limited. Though this problem can be solved if the film thickness of a gas-permeable membrane is reduced and the area of the responsive portion of the sensor is increased, the sensor is readily influenced by flow of the specimen solution, and this is a cause of response drift. Accordingly, in a case where a small amount (on the order of 1 .mu.l ) of a specimen solution is to be measured, it is preferred that the form of the sensor be such that the area having the oxgyen-reducing function be very small (on the order of square microns). In particular, in a case where a miniature sensor is directly inserted into a living body to continuously measure the partial pressure of oxygen in a solution containing a substance to be reduced, it is preferred that the oxygen sensor be one in which coexisting substances are excluded and only the oxygen is acted upon by the oxygen-reducing reaction efficiently by way of an oxygen-reducing membrane. It is especially desired that the sensor have a very small area also from the standpoint of excluding the coexisting substances.
With an oxygen sensor in which the gas-permeable membrane is further coated with an enzyme-fixing membrane, time is required for the permeation of oxygen gas and response time (a 90% response) takes more than one minute.
Thus, there is increasing demand for a quick-response, miniaturized film-coated sensor in which a solid film coating is readily controlled, and in which there is little drift.
Furthermore, when measurement is performed in a blood component, accurate measurement is made impossible owing to adhesion of protein to the surface of a platinum electrode.